Amelanotic malignant melanoma: two collision tumors presenting as basal cell carcinoma and atypical fibroxanthoma.

Collision (contiguous) tumors of the skin can result in misleading clinicopathological presentations, and the choice of appropriate diagnostic techniques may prevent incomplete diagnosis and management. We report 2 cases of collision tumors involving amelanotic malignant melanoma of the back. One patient is a 79-yr-old male with an 8.7 x 5.5 x 4.5 cm polypoid lesion that on shave biopsy was diagnosed as basal cell carcinoma. Subsequent excision showed that the lesion was largely composed of amelanotic melanoma underlying a relatively small and thin basal cell carcinoma, and this probably would have been demonstrated in a punch (rather than shave) biopsy. The other patient is a 71-yr-old male with a 1 cm exophytic lesion on the back, which was determined microscopically to be melanoma, and a 0.6 cm papule on the back. This lesion was composed of 2 distinct contiguous neoplastic infiltrates, the predominant component being an atypical fibroxanthoma and the smaller component an amelanotic melanoma (primary vs metastatic), with diagnostic confirmation requiring multiple immunohistochemical stains.